1. Field of the Invention
The present invention relates to an optical element for incorporation into optical transmission means such as tele-cables, which element is of the type comprising an optical fiber of glass or resin possibly provided with a thin protective layer applied closely around the fiber as well as a coating with a substantially circular cross section applied essentially coaxially closely around the fiber. The invention further relates to a transmission means comprising such optical elements.
2. Description of the Prior Art
It is known to prepare tele-cables with optical fibers where the fibers are placed freely movable in the cables in longitudinal spaces with cross-sectional dimensions considerably bigger than the fiber diameter, cf. for example the Swedish patent application No. 75,08599-3 corresponding to DT-OS No. 25 28 991 and DT-OS No. 25 05 621 where a fiber is placed freely movable in a first jacket which further is placed freely movable in a second jacket.
It is also known to establish a twisted placing of the fibers in such spaces. Furthermore, it is known to adhere fibers in undulated paths between two plastic bands which then are wound around a massive support wire, cf. U.S. Pat. No. 3,937,559 (DT-OS No. 24 24 041). Moreover, cables are also known where the fibers are wound helically around a soft support layer placed around a central reinforcing member, cf. U.S. Pat. No. 3,883,218 (DT-OS No. 23 55 854).
DT-OS No. 25 19 050 describes an optical cable with prestressed strength members which are shaped so as to form compartments in which optical fibers are placed without tension, and after relief of the member a fiber overlength is obtained resulting in an improved elongation at break.
DT-OS No. 24 19 798 describes optical fibers of the type where the fiber itself consists of a core with a high refractive index and a jacket with a lower refractive index, and these fibers are characteristic in that the sheath is surrounded by at least one additional sheath whose heat extension coefficient is lower than the extension coefficient for the internal sheath or the combination of core and sheath
According to prior art a compressive stress can with advantage be produced in the sheath in the preparation of such fibers. However, the "sheath" in question constitutes an integral part of the fiber, and is thus not a coating. In respect of massive fibers a sheath of material is applied on the mentioned first sheath, the heat extension coefficient of which material is lower than the subjacent layer or layers. This outer sheath will thus be exposed to compressive stress and the subjacent layer therefore to tensile stress.
In respect of the special type of fibers which have a fluid core surrounded by a massive sheath being in itself free from tension, the sheath is, according to the publication, surrounded by a further sheath having a higher heat extension coefficient which therefore is exposed to tensile stress, and the first sheath is exposed to compressive stress. However, such coated fibers are said to be very sensitive to tensile and bending forces for which reason they must according to the publication be provided with a further sheath whose heat extension coefficient is lower than that of the other sheath. Consequently, the first and third sheath are exposed to compressive stress while the second sheath is exposed to tensile stress.
In other words, the aim according to the publication is to expose the outer sheath to a constant compressive stress as this is said to give the best mechanical strength.
From the U.S. Pat. No. 3,980,390 it is known to improve the mechanical properties of the fiber such as tensile strength at break, elongation at break and minimum bending diameter by applying a two-layer polymer coating.
It is a common aim of these known cable types to prevent as far as possible the mechanical impacts at tension or bending from affecting the optical fibers and especially prevent detrimental tensile impacts.
The object of the present invention is to provide constructional characteristics whereby it is possible to a higher degree than heretofore to avoid deterioration or destruction of the optical fibers with respect to their transmission capability as a consequence of mechanical impacts such as tension, bending, torsion and vibration. In this connection it must be remembered that deterioration or even destruction of the transmission capability of an otherwise perfect fiber can be expected, if due to one or more defects its light conducting interior is narrowed or bent even at an extremely small part of a cable section where the magnitude of the extent of the defect is only a fraction of a millimeter for which reason such defects are referred to as microcracks or microbendings. It is obvious that a tensile force affecting the optical fiber will increase the possibility of cracks the bigger the tensile force is. In the heretofore known constructions of tele-cables with optical fibers efforts have been made, as previously stated, to reduce the size and risk of tensile stresses in the optical fibers.